The use of a single resistance wire formed into a helical coil for use in electric resistance heating either for heating moving air, for radiant heating or for convection heating is well known in the prior art. In one type of heater, the resistance coils are energized to heat air passing over the coils, the heated air then being directed in a particular manner for heating purposes. One application using such a heater is an electric clothes dryer.
Examples of open coil heaters are found in U.S. Pat. Nos. 5,329,098, 5,895,597, 5,925,273, 7,075,043, and 7,154,072, all owned by Tutco, Inc. of Cookeville, Tenn. Each of these patents is incorporated by reference in its entirety herein. One type of an open coil electric resistance heater is a two stage heater described in U.S. Pat. No. 7,075,043. A side view of this type of heater is shown in FIG. 1 and designated by the reference numeral 10. The heater 10 has two heater elements 10a and 10b, optimally for use in a clothes dryer. The elements 10a and 10b are supplied with electricity via terminals 12 extending from the terminal block 28. The heater elements 10a, 10b are supported by a metal plate 14, which in turn supports a plurality of support insulators 16, typically made of ceramic material and which are well known in the art. The support insulators 16 support and isolate coiled portions of the elements, 10a and 10b, during operation of the heater.
The heater 10 includes opposing sidewalls (one shown as 6 in FIG. 1), wherein projections in the plate 14 extend through slots 20 in the sidewall 6 to allow the sidewalls to support the plate.
Each of the electric heater elements, 10a and 10b, is arranged in series of electrically continuous coils which are mounted on the plate 14 in a spaced-apart substantially parallel arrangement. Each heater assembly 10a and 10b is arranged substantially equally and oppositely on both sides of the plate. Crossover portions 22a and 22b of each heater element 10a and 10b are provided wherein each crossover links one coil of each of the elements mounted on one side of the plate 14 with another coil of the same element found on the other side of the plate.
Electricity is supplied to the heater assembly through the terminal block 28. The heater elements, 10a and 10b, are arranged so that the terminal connector portions or wire leads 32 and 34 which extend from an end 38 of each of the mounted coil sections to the terminal block are as short as possible. This aids in eliminating or reducing the need for supporting the connector portions. For the longer runs, the wire leads, 32 and 34, are partially enclosed with an insulating member 36. The insulating member 36 may be formed from any type of insulating material suitable for this purpose, e.g., a ceramic type. The insulating member is generally tubular in shape and rigid.
FIG. 2 shows a typical plate 14 that supports the insulators 16. The plate 14 has a pair of cutouts 24 and 26, wherein cutout 24 accommodates the crossover portion of the heating element and allows for installation of a standoff for support of the heating element, with cutout 26 allowing for a standoff mounting. Cutout 28 accommodates the mounting of the terminal blocks.
The plate 14 also has a series of cutouts 30, which allow for mounting of the insulators 16. The cutouts 30 are elongated in shape to allow for insertion of the insulators in a first orientation and then twisting of the insulators for engagement with the plate for secure mounting. It should also be noticed that the cutouts are generally aligned in a longitudinal path along the plate 14. The resistance wire coil is supported along the path, designated as A in FIG. 2, by the insulators mounted in the cutouts 30. The path A also defines the flow path of the air passing over the open coil. The insulators are designed with some combination of notches or with notches and arms or protrusions to mate with the cutouts 30 in the plate 14 holes and may be retained by tabs in the plate 14. The mounting of these insulators is well known and a further description is not deemed necessary for understanding of the invention. The plate 14 is affixed to the ductwork in such a fashion as to assure airflow over the heater coils, maintain required electrical spacing and provide for the routing of electrical power. The actual cross-section of the ductwork depends upon requirements of the particular application. While FIG. 1 is representative of a heater using a rectangular cross section for air flow, the ductwork can be circular or another cross sectional shape as well.
The manufacturing of appliances and equipment, especially clothes dryer manufacturing, often requires that open coil electric resistance heaters be mounted in a heater duct. As the clothes dryer operates, the temperature experienced by the heater in a duct will increase and decrease over several hundreds of degrees. It is natural for the materials of which the heater and duct are made to undergo expansion and contraction during the thermal cycling of the dryer. Generally the metal plates supporting the coil, e.g., the plate 14 of FIGS. 1 and 2, will undergo some degree of oxidation with time.
The prior art heaters made with heater coils supported by insulators as illustrated above are designed so as to accommodate differences in the thermal expansion rate of the metal plate as compared to the thermal expansion of the insulators which support the heater coils plus accommodate any build up of oxide on the metal plate. The method used in the prior art is to create the holes and tabs, i.e., the cutouts, in the metal plate and design the slots and arms of the insulators so there will be sufficient clearance to accommodate expansion, contraction and metal oxide buildup on the metal plate. Expansion, contraction and metal oxidation build-up can then occur without damage to either the insulator or to the metal plate. If such accommodations are not made, there may be conditions develop in which the insulators can actually break or the metal plate can crack or both conditions may occur.
A consequence of the necessary clearance between the cutouts in the metal plate and the mating slots in the insulators means that under certain conditions of operation vibration of the equipment may occur and the heater itself will vibrate which in turn results in the insulators vibrating against the metal plate. This means the necessary looseness of the insulators in the metal plate may create noise during operation. If the noise is high enough, the user of the equipment, e.g., the user of a clothes dryer in a home, may determine the noise level is objectionable. The objectionable noise level is considered a problem with the prior art.
Another problem with open coil electrical resistance heaters is the configuration of the open coil resistance wire heating element. That is, heaters are often designed with the heater coils configured so as to be made up of a number of straight sections, the axis of each section running parallel to the axis of the air duct. This is best seen when referring to FIG. 2, wherein the axis of the open coil would be parallel to the path of air travel. Other configurations, such as “figure 8” arrangements (see U.S. Pat. No. 4,268,742) or short straight sections running perpendicular to the air flow, which is depicted in FIG. 1 (see U.S. Pat. No. 5,329,098) also exists. As air is either drawn or forced through the duct to be heated by the heating element, downstream heater coil convolutions in any given straight section are shadowed by upstream heater coil convolutions from that same given straight coil section. This means the heater coil being shadowed operates at temperatures higher than when compared to a no shadowing condition. Coil shadowing is considered to be undesirable. Heaters with a “figure 8” coil arrangement will partially address the coil shadowing. If attempts are made to design the straight coil sections at an angle to the axis of the duct, it is necessary to reduce the spacing between coil passes, which is undesirable.
As with all open coil heaters, for a heater made with heater coils supported by insulators, which are in turn supported by a metal plate, it is desirable to have the heater coils arranged so that as much of the duct cross section as possible is filled or covered by heating element material so as to maximize heat transfer to the air stream. The straight coil pass method is not always the best for this because of the shadowing problem. The “figure 8” method of arranging the heater coil is one method used to increase the portion of the duct covered by heater coil. When the length of heater coil is sufficiently long or for multiple coils, as in multi-stage heaters, the “figure 8” arrangement is not a suitable choice as additional space to accommodate the arrangement may not be available.
Therefore, a need has developed to provide improved open coil electrical resistance heaters. The present invention responds to this need with an improved heater configuration that overcomes the problems noted above, especially minimizing noise due to vibration of insulators during heater operation, minimizing shadowing, and increasing the exposure of the coil to the air to be heated.